Doctors have long observed odd, circular cells in semen. They resemble undeveloped sperm, but scientists have been unable to work out how or why they appear—until now.

Whenever a sperm sample is checked, healthy swimmers are counted along with the mysterious round cells. They’re often assumed to be underdeveloped sperm, but sometimes classified as white blood cells, reports New Scientist.

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Now, researchers led by Gianpiero Palermo from Weill Cornell Medical College in New York have analyzed the sperm from 4,800 men to better understand exactly what the round cells are. Analysis revealed that around a quarter were white blood cells, while the remainder were cells with a half-set of chromosomes—like sperm. More interestingly, though, these cells also contained a protein that helps sperm to develop, suggesting that they are immature sperm cells in arrested development, that have found their way into the semen.

How do they get there? Well the team also noticed that the number of round cells appears to rise in the winter. A healthy dose of speculation and a little analysis later, and the team has discovered that the number of round cells seemed to be strongly correlated with cases of influenza. The team reckons that the flu virus may disrupt the production of sperm—hence the rise in rogue round cells. But don’t worry: they also say that sperm production quickly returns to normal, and that the cells themselves are nothing to worry about anyway. Phew. [New Scientist]